Calibration of non-contact current sensors

ABSTRACT

Calibration of a non-contact current sensor provides improved accuracy for measuring current conducted through a conductor such as an AC branch circuit wire. In a calibration mode, a predetermined DC current is injected through a conductor integrated in the non-contact current sensor. The magnitude of the magnetic field is measured using a sensing element of the non-contact current sensor. Then, when operating in measurement mode, a current conducted in a wire passing through the non-contact current sensor is determined by correcting the output of the non-contact current sensor using the result of the measurement made in the calibration mode.

This U.S. patent application is a Continuation of U.S. patentapplication Ser. No. 13/159,536, filed on Jun. 14, 2012, and claimspriority thereto under 35 U.S.C. 120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to power measurement systems, and morespecifically to calibration of a non-contact sensor that includes amagnetic field sensor for detecting the current in a wire of a powerdistribution system.

2. Description of Related Art

A need to measure power consumption in AC line powered systems isincreasing due to a focus on energy efficiency for both commercial andresidential locations. In order to provide accurate measurements, thecharacteristics of the load must be taken into account along with thecurrent drawn by the load.

In order to determine current delivered to loads in an AC powerdistribution system, and in particular in installations already inplace, current sensors are needed that provide for easy coupling to thehigh voltage wiring used to supply the loads, and proper isolation isneeded between the power distribution circuits/loads and the measurementcircuitry.

Non-contact current sensors provide for easy installation, circuitisolation and other advantages in power measurement systems. However,such sensors may vary in fabrication, installation and application andthe relationship between the measured current and the output of thesensor may not be well established.

Therefore, it would be desirable to provide techniques for calibratingnon-contact current sensors and systems including such calibration.

BRIEF SUMMARY OF THE INVENTION

The invention is embodied in a calibration method and sensors andsystems including calibration circuits and other calibration featuresthat implement the techniques of the present invention.

The method and system select between a calibration mode and ameasurement mode in a circuit connected to a non-contact current sensor.In the calibration mode, the system conducts a predetermined DC currentin a conductor included in the current sensor and measures the magnitudeof a magnetic field generated by the predetermined current using theoutput of a sensing element within the current sensor. In themeasurement mode, the system measures a magnitude of the magnetic fieldgenerated by current conducted in a wire passing through the non-contactcurrent sensor using an output of the sensing element and corrects theresult in conformity with a result of the measurement made in thecalibration mode.

The foregoing and other objectives, features, and advantages of theinvention will be apparent from the following, more particular,description of the preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives, and advantages thereof,will best be understood by reference to the following detaileddescription of the invention when read in conjunction with theaccompanying Figures, wherein like reference numerals indicate likecomponents, and:

FIG. 1A and FIG. 1B are isometric views and FIG. 1C is a cross-sectionview of a sensor according to an embodiment of the present invention.

FIG. 2 is an electrical block diagram illustrating circuits forreceiving inputs from sensors according to embodiments of the presentinvention.

FIG. 3A is an isometric view and FIG. 3B is a cross-section view of asensor according to another embodiment of the present invention.

FIG. 4A is an isometric view and FIG. 4B is a cross-section view of asensor according to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention encompasses current sensing systems and methods,as well as sensor devices that perform or are adapted for, calibrationof a non-contact current sensor by using a voltage sensing conductor toinject a predetermined current in a calibration mode and using thecurrent sensor to obtain a current sensor calibration value. The currentmay be an AC current or a DC current. In measurement mode, the voltagesensing conductor is used to measure the magnitude and/or the phase ofthe voltage on a wire inserted in the non-contact current sensor bymeasuring the electrostatic field generated by the wire. Duringsubsequent measurements of current through a wire inserted in thenon-contact current sensor, the output of the current sensor iscorrected using the current sensor calibration value. Further, failureto detect an output from the current sensor at a threshold level inresponse to injection the predetermined current can be used as a sensorfailure indication. In other operating modes, an adjustable current canbe injected using the voltage sensing conductor and the linearity(current step to sensor output voltage/current step) determined ormapped for the sensor, and also a saturation current level can bedetermined as a current level at which the output of the current sensorstarts to lose linear relationship with the injected current.

Referring now to FIGS. 1A-1C, a sensor 10A that may be used in a systemin accordance with an embodiment of the present invention is shown. Aplastic sensor body 12A encloses a current sensor that providesinformation about a magnitude and phase of a current passing through awire 3 around which sensor body is detachably secured as shown in FIG.1B. A latch 23 secures a top portion and a bottom portion of sensor body12A together, along with a hinge formed on sensor body 12A at anopposite side from latch 23. A current sensing portion of sensor 10A isformed by three ferrite pieces 14A, 14B that form a ferrite cylinderaround wire 3, when sensor body 12A is closed. Top ferrite piece 14Aforms a half-cylinder, while ferrite pieces 14B define a gap betweenferrite pieces 14B and in the circumference of the ferrite cylinder, inwhich current sensing element 17, which is generally a semiconductormagnetic field sensor, such as a Hall effect sensor, is disposed.Current sensing element 17 is shown as having interface wires 15extending from its body, but other types of terminals may be used as analternative manner of providing connections to current sensing element17. An aperture is formed through sensor body 12A to receive currentsensing element 17. A voltage sensor is formed by a wire 15A thatextends through sensor body 12A and continues outside of sensor body forconnection to external processing circuits. However wire 15A mayalternatively terminate on a terminal or other suitable electricalconnector disposed on sensor body 12A. The voltage sensor provides an ACwaveform that is at least indicative of the phase of the voltage on wire3 and may be calibrated to provide an indication of the magnitude of thevoltage if needed. FIG. 1C illustrates a cross section of sensor 10Ashowing details of the relationship between current sensing element 17,ferrite pieces 14A and 14B, and voltage sensing conductor wire 15Awithin sensor body 12A. In the present invention voltage sensingconductor wire 15A is used to provide calibration of current sensor 10Aas will be described in further detail below.

Referring now to FIG. 2, a circuit in accordance with an embodiment ofthe invention is shown in a block diagram. The circuit of FIG. 2implements a system for measuring current that receives input from thecurrent/voltage sensors of FIGS. 1A-1C or other current sensorsincluding current sensors 3A-3B and 4A-4B as described below. Interfacewires 15 from current sensing element 17 provide input to a currentmeasurement circuit 108A, which is an analog circuit that appropriatelyscales and filters the current channel output of the sensor. The outputof current measurement circuit 108A is provided as an input to ananalog-to-digital converter (ADC) 106, which converts the current outputwaveform generated by current measurement circuit 108A to sampled valuesprovided to a central processing unit (CPU) 100 that performs powercalculations in accordance with program instruction stored in a memory104 coupled to CPU 104. Alternatively, current measurement circuit 108Amay be omitted and current sensing element 17 may be connected directlyto ADC 106. The power usage by the circuit associated with a particularcurrent sensor can be determined by assuming that the circuit voltage isconstant (e.g., 115V rms for electrical branch circuits in the U.S.) andthat the phase relationship between the voltage and current is aligned(i.e., in-phase). However, while the assumption of constant voltage isgenerally sufficient, as properly designed properly distribution systemsdo not let the line voltage sag more than a small amount, e.g., <3%, thephase relationship between voltage and current is dependent on the powerfactor of the load, and can vary widely and dynamically by load and overtime. Therefore, it is generally desirable to at least know the phaserelationship between the branch circuit voltage and current in order toaccurately determine power usage by the branch circuit. An input/output(I/O) interface 102 provides either a wireless or wired connection to alocal or external monitoring system.

The voltage sensor wires 15A from each end of sensor 10A are provided toa selector S1 that is controlled by a control signal measure providedfrom CPU 100. When control signal measure is asserted, the circuit is inmeasurement mode, and the voltage sensor wires 15A from each end ofsensor 10A are coupled together and provided to an input of voltagemeasurement circuit 108B, which is an analog circuit that appropriatelyscales and filters the voltage channel output of the sensor. Azero-crossing detector 109 may be used to provide phase-only informationto a central processing unit 100 that performs power calculations,alternatively or in combination with providing an output of voltagemeasurement circuit to an input of ADC 106. Alternatively, voltagemeasurement circuit 108B may be omitted and the corresponding output ofselector S1 connected directly to ADC 106. When control signal measureis de-asserted, the circuit is in calibration mode, and voltage sensorwires 15A from each end of sensor 10A are coupled to a current source101 that generates a predetermined calibration current through voltagesensor wire 15A. Also in calibration mode, a current measurement is madeto determine an indication of the magnetic field generated by thecurrent passing through voltage sensor wire 15A as indicated by theoutput of current measurement circuit 108A, which receives the output ofthe current sensor. Since the predetermined current level generated bycurrent source 101 is known, the output of current measurement circuit108A provides a scale factor that can be used to correct subsequentmeasurements of current by current sensor 10A, e.g., the current passingthrough wire 3. Current source 101 may be a DC current source, so thatCPU 100 can use a low-pass filter or integrator algorithm to remove ACnoise from the calibration measurement, or alternatively, current source101 may be an AC current source and a bandpass filter or algorithm canbe used to remove other noise and offset from the measurement. The DCcalibration measurement may be performed while the current is beingpassed through wire 3.

An exemplary set of measurements provide illustration of a calibrationtechnique in accordance with the above-described embodiment of theinvention. In calibration mode, if the predetermined current levelgenerated by current source 101 is given by I_(CAL) and the outputvoltage of voltage measurement circuit 108B is given by V_(CAL), then,as long as sensor 10A is linear and all of the circuits in FIG. 2 remainlinear, the output of voltage measurement circuit 108B for an unknowncurrent level I_(UNK) is given by:

V _(MEAS) =I _(UNK)(V _(CAL) /I _(CAL))

Therefore, unknown current level I_(UNK) can be determined from:

I _(UNK) =K·V _(MEAS),

where calibration value K=I_(CAL)/V_(CAL). Further, if in calibrationmode V_(CAL) does not exceed a predetermined threshold, the system canindicate a sensor failure, which may be a connection failure in one ofwires 15 or voltage sensing conductor 15A, or may be a failure of sensor17 or the measurement circuit. Further, while the above equations assumelinear behavior, current source 101 may be an adjustable current sourcethat in a linearity measuring mode is adjusted according to a controlvalue Adjust, which controls the magnitude of the current injected involtage sensing conductor 15A when control signal measure isde-asserted. A table of calibration values may be stored and/orcoefficients may be determined to form a piecewise linear or otherapproximation that permits non-linear computation of I_(UNK) fromV_(MEAS). A saturation level may be detected for sensor 10A whenincreases in the adjustable current level commanded by control valueAdjust no longer lead to expected increases in measured voltage levelV_(MEAS). For example, operation of the sensing system may be restrictedto current levels that have less than a predetermined error due tonon-linearity in the sensor, or the measurement range may extend tolevels at which correction has high error due to the measured voltagelevel V_(MEAS) changing by small fractions of the value expected ifsensor 10A were linear.

Once the system is calibrated, when power factor is not taken intoaccount, the instantaneous power used by each branch circuit in a powerdistribution can be computed as:

P _(BRANCH) =V _(rms) *I _(meas)

where V_(rms) is a constant value, e.g. 115V, and I_(meas) is a measuredrms current value, such as an rms current value computed by the circuitof FIG. 2 from the calibrated current measurements described above.Power value P_(BRANCH) may be integrated over time to yield the energyuse. When the phase of the voltage is known, then the power may becomputed more accurately as:

P _(BRANCH) =V _(rms) *I _(meas)*cos(Φ)

where Φ is a difference in phase angle between the voltage and currentwaveforms. The output of zero-crossing detector 109 may be compared withthe position of the zero crossings in the current waveform generated bycurrent measurement circuit 108A and the time ΔT between the zerocrossings in the current and voltage used to generate phase difference Φfrom the line frequency (assuming the line frequency is 60 Hz):

Φ=2Π*60*ΔT

In general, the current waveform is not truly sinusoidal and the aboveapproximation may not yield sufficiently accurate results. A moreaccurate method is to multiply current and voltage samples measured at asampling rate much higher than the line frequency. The sampled valuesthus approximate instantaneous values of the current and voltagewaveforms and the energy may be computed as:

Σ(V _(n) *I _(n))

A variety of arithmetic methods may be used to determine power, energyand phase relationships from the sampled current and voltagemeasurements.

Referring now to FIGS. 3A and 3B, a sensor 10B in accordance withanother embodiment of the invention is shown. Sensor 10B is similar tosensor 10A of FIGS. 1A-1C, so only differences between them will bedescribed below. In sensor 10B, a voltage sensor is included. Metalplates 18E and 18F are disposed within ferrite current sensor portions14E and 14F and provide capacitive coupling to a wire disposed withincurrent sensor 10B, when sensor body 12B is closed. Metal plates 18E and18F may be inserts mechanically secured by sensor shell 12B, or metalfilms bonded to or deposited on the interior surfaces of ferrite pieces14E-14F. A pair of terminals 16A and 16B provide solderable connectionsat the ends of sensor body 12B so that wires may be attached to connectto metal plate 18F. In the illustrated example, metal plates 18E and 18Finclude jogs at their ends in order to provide electrical contactbetween them and ferrite pieces 14E-14F do not make contact as in sensor10A of FIGS. 1A-1C. Therefore, the total circumferential gap in theferrite cylinder is increased slightly. However, in alternativeembodiments, the jogs may be omitted from metal plates 18E and 18F andalternative electrical connection techniques may be employed, such asincluding additional terminals. In calibration mode, metal plates 18Eand 18F conduct the predetermined calibration current in combination.

Referring now to FIGS. 4A and 4B, a sensor 10C in accordance with yetanother embodiment of the invention is shown. Sensor 10C is similar tosensor 10B of FIGS. 3A-3B, so only differences between them will bedescribed below. Each of metal plates 18C and 18D include a separate setof terminals 16A,16B and 16C,16D, which can be connected in parallelpairs, or independently selected for injecting the predeterminedcalibration current during calibration mode.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in form,and details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A circuit, comprising: a current sensor having ahousing for detachably coupling the sensor to a wire; a current sensingdevice integrated in the housing for providing a current sensor outputindicative of an AC current conducted through the wire; a calibrationconductor that does not make electrical contact with the wire andextends through the housing; a DC current source coupled to thecalibration conductor for providing a DC current through the calibrationconductor; and a current measuring circuit that measures the currentsensor output of the current sensing device to obtain a currentcalibration value by filtering and measuring a DC component of thecurrent sensor output, wherein the current measuring circuit furthermeasures an AC component of the current sensor output to obtain an ACcurrent measurement value and adjusts a value of the AC currentmeasurement value in conformity with the current calibration value. 2.The circuit of claim 1, wherein the current measuring circuit measuresthe DC component of the current sensor output to obtain the currentcalibration value while the wire conducts an AC current.
 3. The circuitof claim 1, wherein the calibration conductor is a voltage sensingconductor used to measure an electric potential on the wire when thecircuit is in a measurement mode, and wherein the circuit furthercomprises a selector circuit coupled to the voltage sensing conductorfor selecting between the measurement mode and a calibration mode,wherein in the calibration mode, the selector circuit couples terminalsof the voltage sensing conductor to the DC current source, and whereinin the measurement mode, the selector circuit couples a terminal of thevoltage sensing conductor to a voltage measurement circuit for measuringthe electric potential on the wire.
 4. The circuit of claim 1, whereinthe current measuring circuit comprises: an analog to digital converterhaving an input for receiving the current sensor output from the currentsensing device; and a processing circuit for executing programinstructions that adjust the value of the current sensor output inconformity with a stored value of the current calibration value.
 5. Thecircuit of claim 1, wherein the current sensing device is asemiconductor magnetic field sensor.
 6. The circuit of claim 1, whereinthe current sensor comprises: at least two ferrite cylinder portionsdisposed within the housing, wherein when the housing is coupled to thewire, the wire passes through a central void defined by the ferritecylinder portions extending through a central axis thereof, and whereina gap is defined along a circumference of a cylinder formed by theferrite cylinder portions; and a semiconductor magnetic field sensordisposed within the gap, wherein the current sensor output is a voltageoutput of the semiconductor magnetic field sensor.
 7. The circuit ofclaim 6, wherein the calibration conductor is a voltage sensingconductor used to measure an electric potential on the wire when thecircuit is in a measurement mode, and wherein the circuit furthercomprises a selector circuit coupled to the voltage sensing conductorfor selecting between the measurement mode and a calibration mode,wherein in the calibration mode, the selector circuit couples terminalsof the voltage sensing conductor to the DC current source, and whereinin the measurement mode, the selector circuit couples a terminal of thevoltage sensing conductor to a voltage measurement circuit for measuringthe electric potential on the wire.
 8. The circuit of claim 7, whereinthe voltage sensing conductor is a conductive metal cylinder having aradius smaller than and disposed within the ferrite cylinder portions,wherein the first terminal and the second terminal of the voltagesensing conductors are located at opposing ends of the conductive metalcylinder.
 9. The circuit of claim 8, wherein the conductive metalcylinder is a metal layer deposited or affixed to at least one of theferrite cylinder portions.
 10. The circuit of claim 1, wherein the DCcurrent source is a variable current source that causes an adjustablecurrent to be conducted through the calibration conductor, and whereinthe current measuring circuit measures the current sensor output of thecurrent sensing device and detects when the first output of the currentsensing device is not increasing proportionate to increases in theadjustable current, whereby the current measuring circuit determines asaturation current level of the current sensor.
 11. The circuit of claim1, wherein the DC current source is a variable current source thatcauses an adjustable current to be conducted through the calibrationconductor, and wherein the current measuring circuit measures thecurrent sensor output of the current sensing device and comparesincreases in the current sensor output of the current sensing device toincreases in the adjustable current, whereby the current measuringcircuit determines an indication of the linearity of the of the currentsensor.
 12. The circuit of claim 1, wherein in the calibration mode, thecurrent measuring circuit generates an indication that the currentsensing device has failed if an expected level of the current sensingoutput of the current sensing device is not obtained in response to theDC current.
 13. A method for calibrating a non-contact current sensor,comprising: detachably coupling the non-contact sensor to a wire; firstmeasuring a first magnitude of a first AC magnetic field generated by anAC current conducted in the wire using an output of a sensing elementwithin the non-contact current sensor; conducting a DC calibrationcurrent of predetermined magnitude through a calibration conductorpassing through the non-contact current sensor; second measuring andfiltering a second magnitude of a second DC magnetic field generated bythe predetermined current in the calibration conductor using the outputof the sensing element to obtain a current measurement calibrationvalue; and correcting a result of the first measuring in conformity withthe current measurement calibration value.
 14. The method of claim 13,wherein the first measuring is performed while the AC current isconducted in the wire.
 15. The method of claim 13, further comprising:selecting between a calibration mode and a measurement mode in a circuitconnected to the non-contact current sensor; responsive to selecting themeasurement mode, performing the first measuring and the correcting; andresponsive to selecting the calibration mode, performing the conductingand the second measuring.
 16. The method of claim 14, further comprisingdetachably securing a housing of the non-contact current sensor aroundthe wire, wherein the first measuring measures a first indication of thecurrent conducted in the wire by measuring a magnitude of magnetic fluxin a ferrite cylinder disposed around the wire and secured within thehousing, and wherein the second measuring measures a second indicationof the DC current conducted in the calibration conductor by measuring amagnitude of magnetic flux in the ferrite cylinder.
 17. The method ofclaim 14, further comprising: selecting a saturation current measurementmode; and responsive to selecting the saturation current measurementmode, providing an adjustable current level through the calibrationconductor; fourth measuring the output of the current sensing element;and detecting when a result of the fourth measuring is not increasingproportionate to increases in the adjustable current to determine asaturation current level of the non-contact current sensor.
 18. Themethod of claim 14, further comprising: selecting a linearitymeasurement mode; responsive to selecting the linearity measurementmode, providing an adjustable current level through the calibrationconductor; fourth measuring the output of the current sensing element;and comparing increases in the output of the current sensing device toincreases in the adjustable current to determine an indication of thelinearity of the of the non-contact current sensor.
 19. The method ofclaim 14, further comprising responsive to selecting the calibrationmode, generating an indication that the non-contact current sensor hasfailed if an expected level of the output of the current sensing elementis not obtained in response to the calibration current.
 20. Anon-contact current sensor, comprising: a housing for detachablycoupling the sensor to a wire; a current sensing device comprising atleast two ferrite cylinder portions disposed within the housing, whereinwhen the housing is coupled to the wire, the wire passes through acentral void defined by the ferrite cylinder portions extending througha central axis thereof, and wherein a gap is defined along acircumference of a cylinder formed by the ferrite cylinder portionsintegrated in the housing; a semiconductor magnetic field sensordisposed within the gap; a calibration conductor that does not makeelectrical contact with the wire and extends through the housing; a DCcurrent source coupled to the calibration conductor for providing a DCcurrent through the calibration conductor; and a current measuringcircuit that measures a current sensor output of the semiconductormagnetic field sensor to obtain a current calibration value by filteringand measuring a DC component of the current sensor output while the wireconducts an AC current, wherein the current measuring circuit furthermeasures an AC component of the current sensor output to obtain an ACcurrent measurement value and adjusts a value of the AC currentmeasurement value in conformity with the current calibration value.